The timing just wasn't right. Brohm only put in 2 years. He has kids at Purdue, and committed to Purdue, that would never have been there otherwise.
Brohm has too much integrity to just walk away from an incomplete job. He has his first real recruits, a top 25 recruiting class, coming in. Most of which surely would have gone sonewhere else if he left. Purdue would have been back to square 1.
It wasn't really about money.